Pittsburgh's air-quality levels spiked at dangerous levels for hours to weeks on end, filled with a lethal cocktail of ash, soot, exhaust fumes, metals, aerosols and chemicals, the association said in its ninth annual State of the Air report.

Among the nation's cleanest communities, air-wise, are Salinas and Santa Cruz in California; Fargo, N.D.; and Palm Beach, Fla.

Pittsburgh, the first non-California city claiming the dubious honor, also ranked second on the list of cities with the most year-round particle pollution.

Los Angeles, bumped to No. 2 on the most polluted list while retaining its top spot as the city with the most year round pollution, saw improvements in its air quality, the ALA said.

"The air quality in several cities has improved, but in others, declines in pollution have stalled," said ALA President Bernadette Toomey. "The trends tell us loud and clear that we need to do more to protect Americans from breathing air that's simply hazardous to their health."